Magnetic Properties and Structures of the $\alpha$- and $\delta$-Phases of $p$-NPNN

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We have investigated the crystal structures and magnetic properties of the $\alpha$- and $\delta$-phases of an organic radical $p$-NPNN ($p$-nitrophenyl $\alpha$-nitronyl nitroxide), which yielded the first organic bulk ferromagnet, $\beta$-$p$-NPNN (the $\beta$-phase). The results are compared with those of the $\beta$- and $\gamma$-phases, which show distinct magnetic ordering due to intermolecular ferromagnetic couplings. Common structural features are found in the four phases. Unlike the other phases, the $\alpha$-phase exhibits a weak antiferromagnetic interaction. This is attributable to phenyl–phenyl overlapping found only in the $\alpha$-phase. The crystal structure of $\delta$-$p$-NPNN contains a packing motif similar to that of the $\gamma$-phase. Though the $\delta$-phase exhibits ferromagnetic interactions, it shows no magnetic ordering down to 0.46 K. It is suggested that ferromagnetic two-leg spin ladders are formed in the $\delta$-phase, instead of a two-dimensional network as in $\gamma$-$p$-NPNN. This low dimensionality explains the absence of a magnetic transition.
2003-07-15